1. Field of the Invention
This invention pertains to improving the performance of ceramic metal halide (CMH) lamps by reducing axial heat loss along an electrode or lead wire assembly. More particularly, the invention relates to controlling thermal conduction or axial heat loss along the leg of an arctube, particularly for lower lamp wattages, although the invention may have application in other CMH lamp sizes or other lamps.
2. Discussion of the Art
CMH lamps have become increasingly popular due to significant customer benefits. Traditionally, quartz arctubes have been commonly used in arc discharge lamps. More recently, these are being replaced by CMH lamps that use a ceramic arctube. CMH lamps provide better color uniformity and stability, as well as increased lumens per watt, relative to traditional arc discharge lamps. A ceramic arctube can operate at a higher temperature than a comparable quartz arctube. It also has a reduced rate of sodium loss.
In high intensity discharge lamps, efficacy and lamp performance are affected by the loss of energy by thermal conduction along the legs, or ends, of the arctube. Managing the energy losses is necessary to get optimal lamp performance. Thermal management is accomplished by designing the various parts of the lamp to limit or control the power loss through that component. The leadwire connects the arc tube to mounting frame. Heat from the ceramic body and from the electrode tip are conducted through the lead wire away from the arc tube. Controlling the axial and radial thermal conductivity of the leadwire can significantly affect lamp performance.
This efficacy and performance penalty is more pronounced at lower lamp wattages and smaller arctube sizes. This is believed to result from the inability to reduce the dimensions of the legs in scale with the reduced dimensions of the arc chamber. Limitations of the material such as the strength, wall thickness, opening size or diameter, and the manufacturing processes all impose limitations that impact on the efficacy of the lamp at the lower lamp wattages and smaller arctube sizes. In high wattage lamps, the internal diameter of the leg must be large enough to pass the electrode tips. This also limits how small one could make the lead wire and thus limits the ability to control thermal losses by the conventional means of reducing lead wire diameter.
A standard CMH lead wire has a three piece construction. An electrode tip preferably constructed from tungsten is supported at one end of a shaft or mandrel typically constructed of Molybdenum. The mandrel is axially joined or welded to a niobium outer lead to which the lamp mount is attached. The lead wire assembly is hermetically sealed inside a hollow, cylindrical ceramic leg of the arctube, typically along the length of the niobium section and covering the Niobium-Molybdenum weld. The preferred method of sealing the interior chamber is accomplished through frit sealing; however, it will be appreciated that other sealing processes known in the art could also be used. As noted above, it is desirable to limit the axial heat flux along the arctube leg by designing the leg structure to have a reduced thermal conductivity. It has been observed that axial heat loss by thermal conduction along the lead wire assembly usually exceeds the axial heat loss by conduction down the ceramic leg. Thus it is desirable to address the axial heat loss in either the molybdenum or niobium sections of the mandrel. A significant reduction in the axial heat loss along the mandrel would proportionally reduce the loss of lamp power along the arctube leg.
In a conventional CMH lamp, the molybdenum section includes a relatively large diameter mandrel with a smaller diameter overwind. For example, a General Electric 39 watt CMH lamp has a mandrel diameter along the order of 0.016xe2x80x3. The overwind component is preferably a molybdenum wire and has a dimension along the order of 0.0045xe2x80x3. Thus, the total diameter is on the order of 0.025xe2x80x3 (0.016+2*0.0045). The overwind has traditionally been added to the mandrel primarily to alleviate thermal expansion stresses that exist between the molybdenum and the ceramic leg. As will be appreciated, heat is easily conducted both axially and radially through the mandrel. It has been determined that the axial and radial heat conduction is much lower through the overwind than through the mandrel as a result of the helical geometry of the overwind. On the other hand, the overall diameter of the molybdenum portion, i.e. the mandrel and the overwind, must maintain a snug fit with the inside diameter of the ceramic leg of the arctube. The traditional solution, therefore, is to reduce the overall diameter of the molybdenum. As noted above, this is not possible in some instances due to limitations on the minimum manufactured inside diameter of the ceramic leg or for other reasons such as having minimum clearance for the electrode tips to be inserted into the arc tube.
Accordingly, a need exists to provide a molybdenum section with a minimum mandrel diameter that still adheres to the overall diameter required for the molybdenum section and satisfies manufacturing constraints in the winding of the overwind on to small mandrels.
An improved molybdenum lead wire assembly for CMH electrodes is provided that addresses the thermal conduction concerns along the legs of the arc tube.
In an exemplary embodiment of the invention, a ceramic metal halide lamp includes an envelope having an arc discharge chamber. First and second openings communicate with and extend from the discharge chamber. First and second electrode leads are received in the first and second openings, respectively. First ends of the electrode leads extend into the discharge chamber. The electrode leads each have a reduced diameter mandrel with a large overwind such that the combined component diameter fits snugly inside the ceramic leg.
In another exemplary embodiment, double or multiple overwinds are provided on the small mandrel. Minimizing the diameter of the mandrel and increasing the diameter of the overwind component while keeping the total component outer diameter constant, by either using a single large overwind or multiple, smaller overwinds, beneficially reduces heat loss along the arctube leg opening. For small mandrels, multiple small overwinds may be more easily manufactured.
A principal advantage of the invention is increased efficacy of a CMH lamp.
Another advantage of the invention resides in the reduced axial heat loss. This can allow for a larger arc chamber which generally gives better lumen maintenance and longer life, particularly in low wattage lamps.
Still another advantage of the invention relates to the improved performance of extra low wattage CMH lamps. Since the majority of the halide dose in a CMH lamp resides in the legs of the arc tube, minimizing the axial heat loss from the leg can increase the effective temperature of the halide dose which results in increased color rendering index (CRI) and other performance characteristics of the lamp.
Still another advantage is reduced seal glass temperatures that result in lamps with longer life. Alternatively, this allows the lamp to have shorter legs with the same lamp life, thus allowing the creation of more compact light sources.
Still other advantages and benefits will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.